Self-assembled helical slow-wave structures for high-frequency signals

ABSTRACT

Traveling-wave tube amplifiers for high-frequency signals, including terahertz signals, and methods for making a slow-wave structure for the traveling-wave tube amplifiers are provided. The slow-wave structures include helical conductors that are self-assembled via the release and relaxation of strained films from a sacrificial growth substrate.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under DE-FG02-03ER46028awarded by the US Department of Energy and under FA9550-19-1-0086awarded by the USAF/AFOSR. The government has certain rights in theinvention.

BACKGROUND

A traveling wave tube (TWT) is a beam-wave structure enablinginteraction between an energetic beam of electrons and anelectromagnetic (EM) wave to transfer energy from the electron beam tothe EM wave for amplification. TWTs are used as compact, high-gain,high-power sources of high-frequency radiation in applications such aswireless communications, biomedical imaging, and radar. Central to theamplification process is a slow-wave structure (SWS) that matches thephase velocity of the EM wave to that of the electron beam; thisstructure is some form of meander transmission line or, more commonly, aconductive helix. The slow-wave structure conducts the traveling EM wavealong a pathway whose total length is greater than the axial dimensionalong which the electron beam travels; thus, the component of the EMwave velocity along the axial dimension matches that of the electronbeam.

Conventional TWT structures use a wire helix whose dimensions arelimited by the smallest gauge wire available, the ability to wind thehelix with precision, the ability to support the helix to keep italigned with the electron beam, and the ability to handle and assemblethe helix into the structure. Other methods for manufacturing helicalSWSs rely on high-precision laser manufacturing and wafer bonding.Unfortunately, these methods are not easily scalable to micro-scaledimensions (and thus higher frequencies) and are not mass-producible oninexpensive and large-area substrates.

SUMMARY

TWT amplifiers and methods for making SWSs for TWT amplifiers areprovided.

One embodiment of a traveling wave tube amplifier includes a slow-wavestructure that includes: a cylindrical scaffold comprising a dielectricfilm, the cylindrical scaffold having an interior surface; and anelectrically conductive helix on the interior surface of the cylindricalscaffold, the electrically conductive helix comprising a plurality ofelectrically conductive strips connected end-to-end; an electron gunpositioned to direct one or more beams of electrons axially through theelectrically conductive helix or around the periphery of theelectrically conductive helix; and an electron collector positionedopposite the electron beam source.

One embodiment of a method of making a slow-wave structure includes thesteps of: forming a dielectric support membrane on a device substrate;forming a sacrificial film on a portion of a surface of the dielectricsupport membrane; forming a scaffold film comprising a straineddielectric material on a portion of a surface of the sacrificial film;forming a plurality of parallel, electrically conductive strips on thescaffold film, each of the electrically conductive strips having aleading end and a trailing end, wherein an edge of the scaffold film orthe trailing ends of the electrically conductive strips are attached tothe dielectric support membrane; selectively removing the sacrificialfilm underlying the scaffold film, wherein the scaffold film relaxes androlls into a cylinder, bringing the electrically conducting strips intoan end-to-end arrangement that forms a helix on an interior surface ofthe cylinder; electroplating the surface of the helix with anelectrically conductive material; forming a first electricallyconductive contact in electrical communication with a first end of thehelix; and forming a second electrically conductive contact inelectrical communication with the second end of the helix.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1 is a schematic diagram of one embodiment of a TWT amplifier.

FIGS. 2A-2K illustrate one embodiment of a method for fabricating a SWSfor a TWT amplifier.

DETAILED DESCRIPTION

TWT amplifiers and methods for making SWSs for TWT amplifiers areprovided. The SWSs include helical conductors that are self-assembledvia the release and relaxation of strained films from a sacrificialgrowth substrate. The self-assembly methods enable wafer-levelfabrication of SWSs having very small diameters, including micron-scalediameters that enable the amplification of terahertz signals.

The basic components of one embodiment of a TWT amplifier are shown inFIG. 1. The components include an electron gun 102, positioned to directan electron beam 104 along an axis 106 through an SWS comprising anelectrically conductive helix 108 that spirals around axis 106. Electrongun 102 is composed of a control anode 110, a control grid 112, and acathode 114. The TWT amplifier further includes a signal input coupler116 that introduces an EM signal into helix 108 and a signal outputcoupler 118 that receives the amplified EM signal from helix 108.Steering magnets 120 or electric fields are arranged around helix 108 tofocus and steer electron beam 104, and an electron collector 122 ispositioned along axis 106, opposite electron gun 102, to remove theunused electron beam energy. Helix 108 is housed in a vacuum housing124. An attenuator (not shown) may also be provided along the path ofthe electron beam to isolate the input and output.

When electron beam 104 is emitted from electron gun 102 and acceleratedtoward electron collector 122, the electrons are in close proximity tothe propagating EM wave. The electron beam is directed along the axis ofthe helix either through the center of the helix or outside of thehelix. In some embodiments, multiple electron beams (“beamlets”) areused. These beamlets are directed in circular pattern around theperiphery of the helix. The conductive helix slows the axial phasevelocity of the EM wave to, or below, the speed of the electrons in thebeam. The kinetic energy in the electron beam is coupled into the EMwave, thereby amplifying the EM wave.

FIGS. 2A-2K illustrate an embodiment of one method of making the SWS forthe TWT amplifier of FIG. 1, with a detailed drawing of the final SWS inFIG. 2K. The process begins with a device substrate, as shown in FIG.2A. If the device substrate 202 is to be retained as part of the finalTWT amplifier, it should be composed of a high resistivity material inorder to avoid or minimize signal loss, particularly at highfrequencies. Alternatively, if device substrate 202 will ultimately beremoved from the amplifier, it should be made of a material that is ableto be selectively removed from the structure. Silicon is an example of asuitable substrate material because it can be made with high resistivityusing, for example, the float zone process and because it can beselectively etched using known wet or dry etching techniques.

Next, a support membrane 204 is formed on at least a portion of thesurface of device substrate 202 (FIG. 2B). Support membrane 204 is socalled because it is a relatively thin layer of material that willeventually support the SWS, as described in more detail below. Supportmembrane 204 is composed of a dielectric, high-resistivity material inorder to avoid signal loss. Support membrane 204 can be formed using,for example, chemical vapor deposition, which allows the membrane to begrown to a desired thickness. The thickness of the membrane is notcritical, provided that it allows for adequate heat dissipation andstructural support. By way of illustration, suitable support membranethicknesses include those in the range from sub-micron (e.g., 500 nm to1 μm) up to 20 μm. Optionally, support membrane 204 can be mechanicallypolished or coated with a smoothing layer, such as spin-on-glass, inorder to facilitate the deposition of overlying conductive layers withlow surface roughness.

The support membrane is desirably thermally conductive in order tofacilitate the dissipation of heat generated by the SWS. However, therequirement for thermal conductivity will depend, at least in part, onthe power output of the TWT; a lower power output will generally requirea lower thermal conductivity. Diamond and silicon nitride (Si₃N₄) areexamples of low-loss materials that can be used for the supportmembrane. Because diamond has a higher thermal conductivity, it may be abetter choice for high-power applications.

A layer of sacrificial material, referred to herein as a sacrificialfilm 206, is then formed over at least a portion of the surface ofsupport membrane 204 (FIG. 2C), and a strained scaffold film 208 isformed on at least a portion of the surface of sacrificial film 206(FIG. 2D). Sacrificial film 206 is so called because it will ultimatelybe selectively removed from the structure, thereby releasing overlyingscaffold film 208. Scaffold film 208 is so called because it is arelatively thin layer of material that self-assembles into a cylindricalsupport structure (i.e., a scaffold) upon release from sacrificial film206, as described below. Scaffold film 208 is composed of a dielectricmaterial, which may be, but need not be, the same dielectric materialfrom which support membrane 204 is fabricated and which may be, but neednot be, thermally conductive. The selection of the sacrificial filmmaterial and the selection of the scaffold film material will beinterdependent, as it must be possible to selectively remove (e.g.,etch) the sacrificial layer from the structure. Examples of suitablescaffold film materials includes silicon nitride (Si₃N₄), silicon oxide(SiO and/or SiO₂), aluminum oxide (Al₂O₃), and diamond, for whichgermanium, germanium oxide, silicon, or a photoresist could be used asthe sacrificial film material. Germanium and germanium oxide can beselectively removed with H₂O and H₂O₂, which dissolve both of thesematerials. XeF₂ can be used for the selective etching of germanium,germanium oxide, and silicon. Photoresists can be selectively removedwith acetone.

Scaffold film 208 is characterized by a film length 210 between aleading edge 211 and a trailing edge 212. Film length 210 is chosen suchthat, upon release from sacrificial film 206, scaffold film 208 relaxesand rolls into a cylindrical tube with leading edge 211 abuttingtrailing edge 212. However, leading edge 211 need not come into perfectabutment with trailing edge 212; a small amount of edge overlap or asmall gap between the leading and trailing edges is permissible. Filmlength 210 will depend upon the desired diameter of the SWS. Thethickness and strain in scaffold film 208 can be tailored to provide thedesired SWS diameter for a targeted frequency. By way of illustrationonly, a diamond scaffold film having a thickness in the range from about10 nm to about 20 nm grown on a germanium or photoresist sacrificiallayer could provide a tube having an inside diameter in the range fromabout 0.5 μm to about 2 μm. By way of further illustration, a siliconnitride scaffold film having a thickness in the range from about 20 nmto about 40 nm grown on a germanium sacrificial layer could provide atube having an inside diameter in the range from about 1 μm to about 5μm and a silicon nitride scaffold film having a thickness of about 250nm grown on a germanium sacrificial layer could provide a tube having aninside diameter of about 30 μm.

A plurality of parallel electrically conductive strips 214 is thenformed on scaffold film 208 (FIG. 2E). The electrically conductivestrips will typically be metal strips, such as gold strips. However,other electrically conductive and electroplatable materials, such ascopper or silver, can be used. Electrically conductive strips 214 arecharacterized by a strip length 216 between a leading end 217 and atrailing end 218. Strip length 216 is chosen such that, when scaffoldfilm 208 is rolled into a cylindrical tube, electrically conductingstrips 214 are brought into an end-to-end arrangement that forms anelectrically conducting helix on the interior surface of the cylindricaltube, as discussed in more detail below. In the end-to-end arrangement,the leading end of each strip desirably abuts or slightly overlaps thetrailing end of its neighboring strip, such that each strip forms oneturn of the helix. It is acceptable for a small gap to exist between theleading and trailing edges of the strips. Such gaps can be filled by asubsequent electroplating step, as described below. In some embodimentsof the TWT amplifiers, trailing ends 218 of electrically conductivestrips 214 are attached to support membrane 204, such that the SWSremains tethered to the support membrane along its long edge after therelease of scaffold film 208 from sacrificial film 206. Alternatively,trailing edge 212 of scaffold film 208 can be attached to supportmembrane 204 to tether the SWS to the support membrane.

Electrically conductive strips 214 should be thin enough that they donot interfere with the rolling of scaffold film 208 upon its releasefrom sacrificial film 206. By way of illustration, electricallyconducting strips 214 will typically have thicknesses in the range from2 nm to 50 nm. The width and pitch of the strips can be selected toprovide a helix with the desired EM wave propagating properties. Boththe thickness and the width of the strips can be increased byelectroplating after the self-assembly of the cylindrical tube.

Electrically conductive contacts 213, 215 can be deposited whenelectrically conductive strips 214 are deposited. Alternatively, theycan be formed at an earlier or later stage of the process. Electricallyconductive contacts 213, 215 may be composed of the same material aselectrically conductive strips 214 or a different material. Thesecontacts can act as the signal input coupler 116 and the signal outputcoupler 118 or they can provide a connection to the signal input andoutput couplers.

FIG. 2F illustrates a set-up that can be used for the subsequentelectroplating of a metal onto the electrically conductive strips 214 ina helix. The set-up includes a patterned plating seed 220 that providesa low-resistance path from a current source to electrically conductivestrips 214. In the embodiment shown here, plating seed 220 includes aplurality of contact lines 222 in electrical communication withelectrically conductive strips 214. However, plating seed 220 could beplaced at different locations on the exposed surface of support membrane204, provided it does not cover the locations where substrate etch gaps(discussed below) are to be placed. Plating seed is formed from anelectrically conductive material, such as aluminum, that can beselectively removed (e.g., etched or dissolved) from support membrane204. In addition, the outer surface of the material of the plating seedthat is in contact with the plating solution (i.e., not the portion ofthe surface in contact with the conductive strips) is desirably one thatis resistant to plating by the metal to be electroplated onto the helix.For example, an aluminum plating seed whose surface in contact with theplating solution may be oxidized to resist plating or a plating seed ofanother metal, such as copper could be coated with aluminum oxide toresist plating by gold. Virtually any metal can be used, provided itsexposed surfaces are electrically insulated from the plating solutionand that both the metal and its electrical insulation can be selectivelyremoved from the structure after the electroplating process is complete.

Optionally, at least some of sacrificial film 206 and scaffold film 208can be removed between electrically conductive strips 214 to formelectroplating gaps 224 that will allow the electroplating solution tomove more freely around the helix during the electroplating process, asillustrated in FIG. 2G. In addition, for embodiments of the TWTamplifiers in which the SWS is to be suspended over device substrate202, portions of support membrane 204 may be removed betweenelectrically conductive strips 214 and/or contact lines 222 to form etchgaps 226 (FIG. 2H). Etch gaps 226 allow an etchant to selectively etchaway the material of device substrate 202 underlying the helix in asubsequent etching step.

Next, sacrificial film 206 is selectively etched to release scaffoldfilm 208, whereby strain release in scaffold film 208 causes it to rollinto a cylinder, bringing electrically conducting strips 214 into anend-to-end arrangement that forms an electrically conductive helix 228on the interior surface of the cylinder. The strain in scaffold film 208comprises a biaxial or uniaxial strain gradient across the thickness ofthe film that causes the film to roll upward. The strain gradient can beimparted to the scaffold film in a variety of ways. For example,sacrificial film 206 can be composed of a material that imparts acompressive strain to a scaffold film that is grown thereon.Alternatively, a thermal-expansion mismatch between sacrificial film 206and scaffold film 208 could provide the requisite strain gradient. Insome embodiments of the devices the strain gradient is engineered intothe scaffold film by tailoring the deposition parameters during itsgrowth.

In helix 228, each electrically conductive strip 214 provides one turnof the helix. The formation of the electrically conductive helix 228 isillustrated in FIG. 2I. Scaffold film 208 is not shown in FIG. 2I sothat the structure of helix 228 can be seen.

The helix is then electroplated to increase the thickness of theelectrically conductive strips from which it is constructed and also tofill in any gaps between the leading and trailing ends of the strips toprovide a continuous helical structure. Helix 228 can be electroplatedwith a metal that is the same as, or different from, the metal used toform electrically conductive strips 214. Electroplating is carried outby immersing helix 228 in an electroplating solution and creating avoltage difference between plating seed 220 and a counter electrode (notshown) to induce metal ions in the electroplating solution to depositonto the surface of helix 228.

Increasing the thickness of the helix via electroplating may providebetter heat transfer and lower signal loss for the TWT amplifier andalso allows for the tailoring of the signal propagating properties(e.g., operating frequencies) of the SWS, whereby smaller inner-diameterwave tubes amplify higher-frequency signals. By way of illustrationonly, the plating material can be deposited to thicknesses of up to 10μm or even greater, including plating material thicknesses in the rangefrom 30 nm to 10 μm. Using the self-assembly and electroplatingprocesses described herein, SWSs capable of amplifying terahertz signalswith frequencies in the range from 0.3 THz to 3 THz can be fabricated.Once the electroplating is completed, plating seed 220 can be removed(FIG. 2J). If the electroplating seed is made from aluminum, this can beaccomplished by etching in a piranha solution or commercially availablealuminum etchants.

Optionally, device substrate 202 can be partially or entirely removedfrom the TWT amplifier using, for example, a mechanical polish, wet ordry etching, or a combination thereof. Removing device substrate 202 canbe advantageous because doing so reduces dielectric loading andincreases interaction impedance, which increases the gain of the TWTamplifier. FIG. 2K shows an embodiment of a TWT amplifier in which thedevice substrate material disposed beneath the SWS is removed by etchingthrough etch gaps 226. This creates a suspended area 230 in supportmembrane 204 on which the scaffold tube and helix 228 are supported. Inembodiments of the TWT amplifier in which the device substrate issilicon or silicon nitride and the support membrane is diamond, a XeF₂etch can be used to selectively remove the substrate.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more.”

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

1. A traveling wave tube amplifier comprising: a slow-wave structurecomprising: a cylindrical scaffold comprising a dielectric film, thecylindrical scaffold having an interior surface; and an electricallyconductive helix on the interior surface of the cylindrical scaffold,the electrically conductive helix comprising a plurality of electricallyconductive strips connected end-to-end; an electron gun positioned todirect one or more beams of electrons axially through the electricallyconductive helix or around the periphery of the electrically conductivehelix; and an electron collector positioned opposite the electron beamsource.
 2. The amplifier of claim 1, further comprising a dielectricsupport membrane, wherein the slow-wave structure is attached to thedielectric support membrane along the length of the slow-wave structure.3. The amplifier of claim 2, further comprising a device substrate,wherein the portion of the dielectric support membrane to which theslow-wave structure is attached is suspended over the device substrate.4. The amplifier of claim 1, wherein each electrically conductive stripcorresponds to one coil of the electrically conductive helix.
 5. Theamplifier of claim 1, wherein the cylindrical scaffold comprises siliconnitride, silicon oxide, aluminum oxide, or diamond.
 6. The amplifier ofclaim 2, wherein the dielectric support membrane comprises diamond orsilicon nitride.
 7. The amplifier of claim 6, wherein the electricallyconductive material is a material that can be electroplated.
 8. Theamplifier of claim 7, wherein the electrically conductive helixcomprises gold, copper, nickel, or silver.
 9. The amplifier of claim 8,wherein the cylindrical scaffold comprises silicon nitride, thedielectric support membrane comprises diamond, and the electricallyconductive helix comprises gold.
 10. The amplifier of claim 8, whereinthe electrically conductive helix has an inner diameter in the rangefrom 1 μm to 50 μm.
 11. The amplifier of claim 1, wherein theelectrically conductive helix has an inner diameter in the range from 1μm to 50 μm. 12-20. (canceled)
 21. The amplifier of claim 1, wherein thecylindrical scaffold has an inside diameter in the range from 0.5 μm to30 μm.
 22. The amplifier of claim 1, wherein the cylindrical scaffoldcomprises diamond, has a thickness in the range from 10 nm to 20 nm, andhas an inside diameter in the range from 0.5 μm to 2 μm.
 23. Theamplifier of claim 1, wherein the cylindrical scaffold comprises siliconnitride, has a thickness in the range from 20 nm to 40 nm, and has aninside diameter in the range from 1 μm to 5 μm.